Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising. Typically, a transmission has a housing mounted to the vehicle structure, an input shaft driven by an engine crankshaft, and an output shaft driving the vehicle wheels, often via a differential assembly which permits the left and right wheel to rotate at slightly different speeds as the vehicle turns.
The various speed ratios are typically selected by engaging certain shift elements while other shift elements are engaged. Different types of shift elements have different characteristics and properties. Automatic transmissions typically utilize a number of actively controlled friction clutches or brakes that transmit a torque between elements in response to a control signal such as a hydraulic pressure. A friction clutch or brake can transmit a controlled amount of torque even when the elements are at different speeds. However, friction clutches often transmit some parasitic torque even when disengaged. Positive engagement shift element such as dog clutches are actively controlled to be in either an engaged state or a disengaged state, but do not transfer torque between shafts that have relative rotation. Positive engagement shift elements typically exert less parasitic drag than a friction clutch of the same maximum torque capacity. Passive one way clutches permit relative rotation in one direction but allow relative rotation in the other direction. Some one way clutches can be actively controlled to enter alternative states such as preventing rotation in either direction or allowing rotation in either direction.